Engineering Materials Learning Outcome When you complete this module you will be able to: Describe the mechanical properties of ferrous and non-ferrous engineering materials, plus the effects and purposes of various alloys. Learning Objectives Here is what you will be able to do when you complete each objective: 1. Discuss the mechanical properties of materials. 2. Describe the various types of ferrous materials with respect to their mechanical properties. 3. Describe the various types of non-ferrous materials with respect to their mechanical properties. 1
INTRODUCTION Properties of a metal are the characteristics by which the metal can be accurately identified, or by which its range of usefulness can be determined. Power engineers are concerned with the applications and properties of two main groups of metals; ferrous and nonferrous. The word ferrous is derived from ferrum, which is the Latin word for iron. Ferrous metals include pure iron and alloys of iron. Nonferrous metals contain no iron, or possibly only a trace. Some nonferrous metals are copper, lead, aluminum, zinc, nickel, tin, and magnesium. Serviceability and safety are the ultimate criteria in choosing metals. Knowledge of types, their performance, and preservation are absolutely essential. By knowing to what extent each property exists in a metal, the metal can be used with the assurance that it will meet requirements for a specific application. MECHANICAL PROPERTIES OF METALS Hardness Hardness is the ability to resist wear, abrasion, cutting, and indentation. It may be a surface condition of a metal such as when a metal is subjected to casehardening, or it may be uniform throughout the metal. Resistance to indentation is the basis for a number of hardness tests. A ball indenter, or a cone or pyramid, made of hardened steel or diamond is loaded so that it produces some indentation. Or the indenter may be dropped from some height and the rebound that it undergoes after striking the metal is a measure of the surface hardness of the metal. The most commonly used tests are the Brinell and Rockwell tests. 2
1. The Brinell Tester Fig. 1 shows a Brinell hardness testing machine. The test specimen is placed on top of the jack screw and raised until it touches the tungsten carbide ball. By means of a pump, oil is forced above the plunger to press the ball into the test specimen. The pressure gage gives an approximate indication of the load. To assure no overload, the masses act as a pressure relief system by absorbing pressure. The ball, 10 mm in diameter, is forced into the piece being tested by a standard 3000 kg load, if the material is steel. For nonferrous metals the standard load is 500 kg. The ball will make a circular mark. The diameter may be measured by a low-powered microscope, the surface area of the mark determined, and the hardness calculated: Brinell Hardness Number (BHN) = Load (kg) Area of mark (mm 2 ) Normally the area of the mark is not calculated. Instead, the diameter of the depression is measured in mm and a chart consulted from which the hardness number can be read directly. AJ_1_0_5.jpg P AJ_1_0_3.mov V AJ_1_0_4.mov A Figure 1 Brinell Hardness Tester 3
2. The Rockwell Tester The Rockwell tester uses a somewhat different principle. A 10 kg load is used to hold a 1.6 mm ball on the piece being tested. Another 90 kg load (100 kg in all) is then applied to make the impression. The 90 kg is removed, and the 10 kg left on. The dial of the instrument measures the depth of the mark to 0.002 mm and converts the depth to a hardness reading which is simply read off the dial. For hard materials, a diamond cone is used and the total load is 150 kg. To avoid confusion, readings made with the ball are called Rockwell B readings, and those with the cone are called Rockwell C readings. Brittleness Brittleness is that property of a metal which permits no permanent deformation before breaking. Brittle materials generally break instantly, without any intermediate stage of bending. An example of a brittle material is cast iron. Ductility Ductility is the property of a material that enables it to be drawn out to a considerable extent before rupture, and at the same time to sustain appreciable load. It is sometimes considered to be the ability of a material to be permanently deformed without breaking. Mild steel is a ductile material. Ductile metal may be cold-drawn into wires, as in annealed copper. Plasticity A material is said to exhibit plasticity, or to be plastic, if it is very soft and easily deformed. Examples of plastic materials include wax, lead, and babbitt. Plastic materials have very little elasticity; that is, they do not return to their original shape after the deforming force has been removed. Plasticity is the opposite of brittleness. Elasticity Elasticity is the ability of a material to return to its original shape after any force acting on it has been removed. Elasticity is one of the most important properties from the engineering point of view as it helps to determine the behavior of the material under a load. Materials that are tough and ductile, such as wrought iron, possess a certain amount of elasticity. Materials that are hard and brittle, such as cast iron, have very little elasticity. 4
Malleability Malleability is that property which allows a material to be hammered or rolled into other sizes and shapes. Copper, which was given as an example of a ductile material, is also malleable, because it can be hammered or pounded into various shapes. The malleability of most materials will increase when the material is heated, as when iron or steel are heated before forging. Toughness Toughness is the property that determines whether or not a material will break under a sudden impact or hard blow. This property is also referred to as impact strength, and impact tests are used to determine the toughness of a material. Two commonly used tests are the Izod and the Charpy tests. In the Izod test (Fig. 2), one end of a specimen of the material to be tested is held in a vice and the free end is struck with a hammer. The energy required to break it is measured, and indicates the toughness of the material. Figure 2 Principle of Izod Impact Test The Charpy test is similar, except that the specimen to be tested is supported at each end and struck in the center with the hammer. In the Charpy Test (Fig. 3), a round or square specimen is notched, then broken by a blow from a pendulum. The specimen is struck when the pendulum is at the bottom of its swing, and struck in such a way that the notch tends to be opened up. The amount of energy used in breaking the specimen is measured simply by noting the height to which the pendulum rises after the break. The results are given as the number of joules of energy absorbed. AJ_1_0_1.mov V AJ_1_0_2.mov A 5
Figure 3 Principle of Charpy Impact Test TYPES OF ENGINEERING MATERIALS Iron AJ_1_0_6.jpg P Iron is produced in a blast furnace from iron ore. The iron ore is added to the furnace together with coke (for fuel) and limestone, which combines with the impurities to form slag. The slag floats on top of the molten iron and is drawn off and discarded. The molten iron is drawn off into ladles and cast into molds to form what is called pig iron. Pig iron by itself is of little or no use. It is hard, brittle, and almost impossible to machine. It cannot be worked whether hot or cold. To make it useful, it must be refined further to give one of two broad classes of materials; steel or cast iron. Cast Iron Cast iron is produced by melting pig iron together with some scrap iron in a cupola furnace which is similar to a small blast furnace. The resulting molten iron contains 2%-4% carbon. If most of the carbon is combined chemically with the iron, then the material is called white cast iron. It can be produced by cooling the molten material rapidly in the mold thus keeping the carbon combined with the iron. If most of the carbon is mechanically mixed with the iron in the form of graphite, then the material is known as grey cast iron. It can be produced by cooling the molten material slowly, which allows the carbon to disassociate and form graphite within the iron. White cast iron is very hard and brittle. It is used for machinery parts that are subjected to excessive wear, such as crusher jaws and grinding mill balls and liners. 6
Grey cast iron is softer than white cast iron and is easily machined. It has good compressive strength and is widely used for machinery bases and supports. Another type of cast iron is malleable cast iron which is produced by annealing (heating and cooling at a controlled rate) white cast iron. The resulting product has increased toughness and ductility, and is used as material for some farm implements, automobile parts, pipe fittings, and tools. Wrought Iron Wrought iron is produced by a process known as a puddling. Pig iron is melted in a furnace, and as it melts, the carbon and other impurities oxidize and leave the iron. As the impurities pass off, the iron forms a plastic mass which is formed into a ball by manipulation of a puddling bar. The ball is then removed from the furnace and squeezed and rolled to remove most of the slag. The result is wrought iron. The important properties of wrought iron are its ductility and resistance to corrosion. It was formerly used quite extensively for boiler tubes, piping, bolts, etc., but has been largely replaced by steel. Steel Steels are alloys of iron and carbon containing less than 2% carbon. If the carbon content is greater than 2%, then the alloy is cast iron. Steels may be divided further into plain carbon and alloy steels. Plain carbon steels are alloys of iron and carbon only. Approximately 90% of the steel manufactured in North America is produced by the open hearth method, with most of the remainder produced by the electric furnace method. AJ_1_0_7.jpg P The open hearth furnace is charged with scrap iron, pig iron, and ore, and the heat is supplied by the combustion gas or oil with preheated air. The very hot gases pass from the furnace through a brickwork checker chamber at one end of the furnace, while the air for combustion enters through another brickwork checker chamber at the other end of the furnace. Every fifteen minutes, the gas and air flows are reversed with the air being heated as it passes through the checker chamber, which in the previous fifteen minutes had been exposed to the hot gases. 7
As the charge in the furnace is heated and melted, the carbon content is reduced to the proper point by oxidation, and in this way carbon steel is produced. If alloy steel is desired, then the required alloying materials are added to the molten carbon steel. Special high alloy steel is frequently produced in electric furnaces where the heat is furnished by electrical arcs. 1. Carbon Steels Carbon steels are grouped according to their carbon content, that is low, medium, high, and very high carbon. (a) Low Carbon Steels Low carbon steels have a carbon content between 0.05% and 0.3% and are commonly referred to as mild steel. (b) Medium Carbon Steels These steels have a carbon content varying between 0.3% and 0.45%. They are strong and hard but not easily welded. Whenever the carbon content exceeds 0.35%, the steel becomes increasingly difficult to weld, due to a greater tendency toward brittleness. (c) High and Very High Carbon Steels The carbon content ranges from 0.45% to 0.75% and from 0.75% to 1.5%, respectively for high and very high carbon steels. They are very strong and hard. Hardness and strength increase with an increased carbon content. Impurities such as phosphorus or sulphur will lower the ductility, malleability, and welding qualities of a steel. High and very high carbon steels respond well to heat treatments. Most of these materials may, in the annealed state, be readily machined. 2. Alloy Steels Alloy steels are carbon steels to which certain elements; such as manganese, nickel, chromium, tungsten, molybdenum, vanadium, or copper, have been added. Each of these elements gives certain qualities to the steel in which they are present. Some of the alloying elements combine with the carbon to form compounds; other elements do not form compounds but remain in solution in the ferrite. In solution means that the elements do not combine with other elements, but are held suspended as crystals in the basic ferrite. 8
The main advantages of alloy steels are the ability to respond to heat treatment, improved corrosion resistance, improved properties at high and low temperatures, and combination of high strength with good ductility. Most alloy steels may be welded, provided the carbon content is within welding range. Generally, these steels require heating before, during, and after welding in order to avoid residual stresses. An example of an alloy steel is Specification SA-335-P22 which is a chromemolybdenum steel used for high temperature steam piping. This material is suitable for severe service because of its high creep strength and resistance to oxidation and corrosion at high temperatures (above 500 C). Creep is slow, permanent stretching of a material under stress, at high temperatures. Some of the more important elements which are added to steel to produce alloy steel, with their effect on the properties of the steel, are the following: (a) Nickel Nickel is a tough, silvery element of about the same density as copper. It has excellent resistance to corrosion and oxidation even at high temperatures. It improves toughness, and prevents brittleness at low temperatures. Nickel steels are especially suitable for the case hardening process for such applications as roller bearings and gears. These steels provide strong, tough cases that are resistant to wear and fatigue. (b) Chromium Chromium resists oxidation caused by hot gases, maintains high strength at elevated temperatures, and increases hardness and abrasion resistance. When chromium is present in amounts in excess of 4.0%, corrosion resistance is greatly promoted. With a minimum of 12% chromium, the steel is called stainless steel. (c) Molybdenum Molybdenum increases hardness and endurance limits of steel, and decreases the tendency towards creep. It also increases the steel s resistance to corrosion. (d) Vanadium Vanadium produces a fine grain structure during heat treatment, promotes hardening ability, and increases ductility. 9
(e) Copper Copper readily combines with many other elements and improves the atmospheric corrosion resistance qualities of the steel. (f) Lead Lead improves machinability. (g) Manganese Manganese increases strength and hardness, promotes high impact strength, and offers excellent resistance to wear by abrasion. (h) Tungsten Tungsten produces a fine grain structure. The alloy retains hardness and strength at high temperatures. NONFERROUS METALS Copper Copper is obtained from copper ore which is smelted and then further refined by electrolysis. It is then made into castings, wire, bars, sheets, plates, tubes, etc. The properties which make copper desirable as an engineering material are its high electrical conductivity, high heat conductivity, high corrosion resistance, high ductility, and toughness. In a power plant, it is used primarily for electrical equipment, and as an alloy in the materials used for heat exchanger tubes, valves, and fittings. Copper Alloys If copper is mixed with other metals, the resulting copper alloy has superior properties to pure copper. For example, copper alloys are stronger, easier to machine, and have better corrosion resistance. The most commonly used copper alloys are various brasses and bronzes. Brasses and bronzes find use as condenser tubes, piping, valves, fittings, and bearing shells. 1. Brass Brasses are primarily a mixture of copper and zinc (up to 40%). Frequently, small amounts of other metals such as lead, tin, nickel, aluminum, and manganese are also included in the mixture. 10
2. Bronze Bronze, an alloy of copper and tin, may also contain zinc to ensure non-porous castings, and lead to improve machining qualities. Additions of up to 1% phosphorus produces bronzes, or bearing bronzes, which are hard but not abrasive. Bronze has a resistance to corrosion approximately equal to that of copper. Aluminum Aluminum is produced by electrolysis from bauxite ore. One of its most important properties is its low density; it is only one third as heavy as iron or steel. It is very malleable and ductile, is a good conductor of electricity, is an excellent conductor of heat, and has high resistance to corrosion. A disadvantage is that in its pure form it has a low tensile strength. Aluminum is usually alloyed with other materials such as copper, silicon, manganese, zinc, nickel, magnesium, and chromium in order to improve its properties. For example, an aluminum alloy may contain 4% copper and 1/2% each of manganese and magnesium. Aluminum alloys are used for internal combustion engine parts, aircraft parts, tubing, water jackets, etc. White Metal White metal is the name given to alloys made up primarily of lead and tin, and in some cases with small amounts of other elements such as antimony, bismuth, silver, or zinc added. They are chiefly used for bearing materials because they are easily melted and cast in the bearing shell, and they have sufficient strength and ductility not to crack or squeeze out under heavy loads. In addition, they are soft enough to wear sufficiently to conform to the shape of the shaft, and they have good thermal conductivity to carry heat away from the bearing surface. Different combinations of the alloying elements are used to produce white metals suitable for various applications. For example, tin-based white metal, also known as babbitt, is composed of 89% tin, 7.5% lead, and 3.5% copper and is used for high speed and light load applications. A lead-based white metal used for slower speeds and heavier loads is composed of 10% tin, 15% antimony, and 75% lead. 11
References and Reference Material For further information on this topic, the following are recommended: 1. Salisbury, J. Kenneth. Kent s Mechanical Engineers Handbook - Power. 12th ed. New York: John Wiley & Sons. 2. Canadian Welding Bureau. Welding Fundamental Principles and Practices. Toronto: Canadian Welding Bureau; 1968. 12